1. Technical Field
The present invention relates to an apparatus for decreasing an inrush current generated when power is put into a transformer, by using a superconducting fault current limiter (SFCL), and a method for deciding the optimal insertion resistance.
More particularly, the present invention relates to an apparatus for optimally decreasing an inrush current of a transformer by deciding the optimal insertion resistance of an SFCL by using the inrush current and voltage before the SFCL is inserted, and a method for deciding the optimal insertion resistance.
2. Discussion of Related Art
In a transformer which is widely used as a key device of electric installation in an industry and at home, an inrush current is generated by the hysteresis characteristic of an iron core within the transformer at the moment when power is put into the transformer.
The inrush current is most influenced by residual magnetic flux which remains in the iron core when turning on and off the power of the transformer. In generally, at the point in time when the power is input, the inrush current increases as the phase of a voltage waveform is close to zero degree and the residual magnetic flux density is greater.
Since the inrush current has rich high harmonics and is from a few to several tens times a normal current in intensity as the case may be, it has seriously adverse effects on the power system, such as malfunction of a protective relay, damage of a transformer, shortening of a transformer life, and the like.
To solve the aforementioned problems, the prior art uses a method for delaying in time for inputting power applied to a transformer by each phase (hereinafter, referred to as a “first method”) and a method for increasing impedance by using a pre-insertion reactor (hereinafter, referred to as a “second method”). An example of the second method is disclosed in detail in Korean Laid-Open Patent Application No. 2004-62938.
However, in the aforementioned conventional first method, whenever power is put into a transformer, setting a proper delay time needs to be always controlled. Moreover, since a speed at which the inrush current decreases is not fast, the power installation is insufficiently protected at the beginning when the inrush current is generated.
Further, in the conventional second method, since the pre-insertion reactor is connected to or separated from a transformer by switching operation, a switching device has an over-voltage. Moreover, when the switching device is used for a long time, it ages to cause a problem regarding reliability of the power system.
Consequently, there has been a demand for a new method which fast decreases the inrush current and needs no special controlling or switching operation when the connection to a transformer is completed.